1. Field of Invention
The present invention relates to a method for adjusting the temperature-dependent property of a surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate. The invention also relates to a surface acoustic wave device utilizing such a method.
2. Description of Related Art
Related art quartz crystal piezoelectric resonators are subject to a temperature-dependent property (i.e., a frequency fluctuation property in association with temperature changes). This property may be expressed by a quadratic function, such as that of tuning fork type resonators and a surface acoustic wave device, or by a cubic function, such as that of AT resonators.
The temperature-dependent property of a piezoelectric resonator is adjusted so that the frequency fluctuation is minimized in the operational temperature range (xe2x88x9240 to +85xc2x0 C.) including a normal operational temperature of 25xc2x0 C. In a piezoelectric resonator having the temperature-dependent property expressed by a quadratic function, the peak temperature of the temperature-dependent property of the piezoelectric resonator (i.e., the temperature that gives an extreme value of frequency) is centered in the operational temperature range so that the frequency fluctuation is minimized. Related art piezoelectric resonators having the temperature-dependent property expressed by a quadratic function have the peak temperature in a range of 0xc2x0 C. to 50xc2x0 C.
The temperature-dependent property of the AT cut resonator, expressed by a cubic function, has a limited frequency fluctuation in the operational temperature range because the temperature at the inflection point is located almost at the center of the operational temperature range.
The surface acoustic wave resonator, typically a SAW resonator or a SAW filter, can utilize an in-plane rotated ST cut quartz crystal plate for the purpose of reducing frequency fluctuation in association with temperature changes.
In the related art, the surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate also is believed to have the temperature-dependent property expressed by a quadratic function, as is the surface acoustic wave device using a related art ST cut quartz crystal plate. Therefore, the peak temperature of the surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate coincides with the center of the operational temperature range so as to minimize the frequency fluctuation associating with temperature changes.
As is described above, in the related art, the surface acoustic wave device using the in-plane rotated ST cut quartz crystal plate is believed to have the temperature-dependent property expressed by a quadratic function.
However, recent studies by the present inventors have proven that the temperature-dependent property is actually expressed by a cubic function having an inflection point nearly at 110xc2x0 C. In general, measurements of the temperature-dependent property are not obtained for temperatures that greatly exceed 110xc2x0 C. This explains why it has not been found that the surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate has the temperature-dependent property expressed by a cubic function. In the related art, an adjustment of the temperature-dependent property of a surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate involves a quadratic function. Therefore, it actually fails to optimize frequency fluctuation in the operational temperature range.
A problem arises that related art adjustment techniques do not substantially optimize the temperature-dependent property of a surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate, since it has the temperature-dependent property expressed by a cubic function, and not by a quadratic function.
In view of the problem described above with regard to the related art, the present invention provides a method for adjusting the temperature-dependent property of a surface acoustic wave device. This method is suitable to the temperature-dependent property expressed by a cubic function of a surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate, and a surface acoustic wave device.
The present invention is based on the finding that the surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate actually has the temperature-dependent property expressed by a cubic function and, therefore, the peak temperature can be shifted from the center of the operational temperature range to further reduce the frequency fluctuation. This is in contrast to the related art where it has been considered to have the temperature-dependent property expressed by a quadratic function, and where the peak temperature is therefore arranged to be located at the center of the operational temperature range.
A method for adjusting the temperature-dependent property of a surface acoustic wave device according to the present invention includes an in-plane rotated ST cut quartz crystal plate having the Euler angles of (0xc2x0, 113-135xc2x0, xc2x1(40-49xc2x0)). The temperature-dependent property of the surface acoustic wave device using the in-plane rotated ST cut quartz crystal plate having the temperature-dependent property expressed by a cubic function is adjusted by rotating it about the inflection point. The range of the Euler angles is determined so that the temperature-dependent property of the surface acoustic wave device using the in-plane rotated ST cut quartz crystal plate has an extreme value and, then, the temperature-dependent property is rotated about the inflection point to minimize fluctuation of the temperature-dependent property in the operational temperature range.
When the Euler angles are (0xc2x0, xcex8, "psgr"), "psgr" is in the following range:
"psgr"=0.3295 xcex8+3.3318xc2x0xc2x11.125xc2x0.
In particular, when the Euler angles are (0xc2x0, xcex8, "psgr"), it is preferred that xcex8=125-128xc2x0, and xcex7 (electrode width/electrode pitch) is 0.3-0.6.
The temperature-dependent property around the inflection point is adjusted by modifying the thickness of the electrode deposits of the surface acoustic wave device. Alternatively, it is adjusted by modifying the rotation of the quartz crystal in a plane about the Zxe2x80x2 axis. Alternatively, it is also adjusted by modifying xcex7 (electrode width/electrode pitch) of the electrodes of the surface acoustic wave device. The surface acoustic wave device of the present invention is manufactured by the method for adjusting the temperature-dependent property of a surface acoustic wave device described above.
As is described above, it has been found by the present inventors that the surface acoustic wave device using an in-plane rotated ST cut quartz crystal plate having the Euler angles of (0xc2x0, 113-135xc2x0, xc2x1(40-49xc2x0)) has the temperature-dependent property expressed by a cubic function. When a range of the Euler angles corresponds to the temperature-dependent property of an extreme value expressed by a cubic function, it is determined within the range (0xc2x0, 113-135xc2x0, xc2x1(40-49xc2x0)). Then, the temperature-dependent property is rotated about the inflection point expressed by a cubic function within the above range to minimize fluctuation of the temperature-dependent property (i.e., frequency fluctuation) in the operational temperature range. In particular, with xcex8 of the Euler angles being 125-128xc2x0 and xcex7 (electrode width/electrode pitch) being 0.3-0.6, the frequency fluctuation can be minimized in the temperature range of xe2x88x9240 to +85xc2x0 C., even taking into account the fluctuation of the temperature-dependent property due to a manufacturing error that has a larger influence on higher frequencies. The temperature-dependent property around the inflection point is modified by adjusting the rotation of the quartz crystal in a plane about the Zxe2x80x2 axis or modifying the thickness or width of the electrode deposits formed on the surface.
The ranges of the Euler angles, in which the temperature-dependent property expressed by a cubic function has an extreme value, have been verified by the present inventors. Therefore, this range can be targeted in modifying the rotation of a plane.